I would go ahead and get two clowns--they're more fun to watch, and as iamwrasseman pointed out, they'll still defend their territory anyway.
Or if you want, you could skip the clowns and just get a pair of firefish (LiveAquaria's Diver's Den offers them sometimes) and a shrimp goby (or a pair of shrimp gobies) with commensal shrimp (these are also available on DD)--that would make for a nice, colorful display of peaceful fish.